ST. AUGUSTINE: THE CHURCH IS FOUNDED UPON THE ROCK WHICH PETER ACKNOWLEDGED.
God never ceases to provide the human race with consolations in misfortune. But in addition to these, in the fullness of time, when he himself knew it should be done, he sent his own only-begotten Son through whom he created all things While remaining God, his Son was to become man and be the mediator between God and men, the man Jesus Christ.
Those who believed in him through baptism were freed from the guilt of all their sins, freed from eternal damnation to live in faith, hope and love. On their journey among the trials and dangers of this world, they received the consolations of God, both in body and in spirit. They were to walk in his sight, keeping to the path which Christ made for them.
But even while walking on this path they were not without sin, since it develops subtly out of human weakness. Therefore Christ gave the saving remedy of charity to help them in their prayers, for he taught them to say: Forgive us our sins as we forgive those who sin against us.
Such is the practice which the Church in blessed hope carries out in this life of suffering. Now the apostle Peter, because of the primacy of his apostleship, stood as a symbol of the entire Church.
In himself he was by nature one man, by grace one Christian, by a more abundant grace an apostle and the chief of the apostles. But Christ said to him: To you I shall give the keys of the kingdom of heaven and whatever you will bind upon the earth will be bound also in heaven and whatever you will forgive upon the earth will be forgiven also in heaven. Now these words applied to the entire Church. In this life it is shaken by various trials, as if by rains, floods and tempests, but it does not fall because it is founded upon the rock from which Peter received his name.
The Lord said: Upon this rock I shall build my Church because Peter has first said: You are Christ, the Son of the living God. The Lord was really saying: I shall build my Church upon the rock which you have acknowledged. For the rock was Christ, and upon this foundation even Peter himself was raised up. Another foundation indeed no one can lay except that which was laid, which is Jesus Christ.
The Church, which is founded upon Christ, received from him the keys of the kingdom of heaven, that is, the power of binding and forgiving sins, in the person of Peter. Therefore this Church, by loving and following Christ, is set free from evil. But this is even more the case with those who fight in behalf of truth even to the death.
St. Augustine, Tract. 124, 5: CCL 36, 684-685